Stone fruit inking cure not found in calcium spraysBy Melissa Hansen Reducing the incidence of surface discoloration on stone fruit won't be easily accomplished. Foliar nutrient, fungicide, and insecticide preharvest sprays may act as sources of contamination for inking development, depending on preharvest application intervals. Surface discoloration, known as inking, has been a growing problem in peach and nectarine varieties around the world. Inking symptoms appear as dark black, dark brown, orange, or tan spots or stripes and are always restricted to the skin, never affecting fruit flesh. Though inking is a cosmetic problem, it causes blemished fruit that are unmarketable. Inking has been linked to fruit abrasion. Tissue damage or abrasion is a requirement for inking, according to Dr. Carlos Crisosto, pomology postharvest specialist for the University of California. "Inking only requires a small amount of abrasion," he said. Dark discoloration develops when fruit are exposed to heavy metal contaminants. The abrasion damage releases anthocyanin/phenolic pigments, which are located in the skin cells, allowing the reaction of these pigments with the heavy metal contaminants. Laboratory research has demonstrated that abraded fruit discs developed inking after exposure to different metallic ion concentrations, including iron, aluminum, copper, tin, zinc, and sodium. Fruit discs which weren't abraded were not discolored when treated with the metallic ions. Research also has found that iron, copper, and aluminum were the worst offenders of those studied in inducing inking on abraded fruit. Iron concentrations as low as ten parts per million (ppm) were enough to induce inking on the abraded disks of all cultivars tested. Contamination from heavy metals can occur within 15 days before harvest, or during harvest and packing operations. Previous studies have demonstrated the role of fungicides in acting as contaminants to induce inking. Many chemicals commonly used by the stone fruit industry contain high amounts of aluminum, copper, iron, and, generally, low concentrations of tin. For example, Rally 40W (myclobutanil) was found to have 3,920 ppm of aluminum and 864 ppm of iron per milligram/kilogram of chemical; Sevin 50W (carbaryl) had 3,742 ppm of aluminum and 276 ppm of iron; and Imidan 50WP (phosmet) had 2,410 ppm aluminum and 291 ppm of iron. Foli-Cal, a foliar calcium, was high in copper at 1,960 ppm, though relatively low in iron, tin, and aluminum. Calcium sprays Though foliar calcium sprays have proved beneficial on apples, researchers in California have been unable to show any benefit to peach and nectarine quality from calcium applications. Repeated calcium foliar spray applications--up to eight times--failed to reduce surface discoloration in the peach and nectarine trials. Flesh firmness, soluble sugar concentrations, and internal breakdown were not affected by any of the calcium sprays. The calcium sprays failed to increase calcium concentrations in the flesh, therefore, a clear relationship couldn't be established between fruit calcium concentration and fruit flesh firmness at harvest or after several weeks in storage. Crisosto suspects that calcium sprays applied within a week of harvest could even be an inking precursor. Several new formulations of foliar calcium sprays were introduced last year; however, he found no benefits to stone fruit from the formulations under California conditions. "I wouldn't recommend calcium sprays as a way to reduce incidence of inking," he concluded. How to reduce inking Crisosto noted that higher-maturity fruit--fruit picked late--has potential for increased inking because it is more susceptible to abrasion. Rubbing and vibration that occurs during harvest and packing contribute to fruit abrasion. University of California (UC) researchers have come up with some things growers can do to minimize inking incidence: 1) Reduce abrasion damage by handling fruit gently, avoiding long hauling, and keeping harvest containers free of dirt. 2) Reduce contamination of fruit from heavy metals by keeping equipment clean, avoiding dust contamination on fruit, checking water quality for heavy metals, and avoiding foliar-nutrient sprays containing heavy metals within 22 days before harvest (DBH). A general DBH guide to follow to reduce inking: apply Benlate (benomyl) within 12 DBH; Rovral (iprodione) 7 DBH; Funginex (triforine) 3 DBH; and Ronilan (vinclozolin) 1 DBH; 3) In case of a possible inking situation, delay packing of peaches or nectarines for 48 hours. This gives inking a chance to develop and will help sorters detect inking damage during packing. Crisosto and other UC researchers are encouraging chemical manufacturers to develop preharvest application intervals to help growers avoid inking. This will help in the short term. In the long term, chemical manufacturers are asked to identify and remove sources of contamination from products that may contribute to inking. *
| ||||